Does your sales process include submitting proposals? Does the proposal include a road map of the solution to your prospect’s problem?
Your prospect could take your expertise and shop it to the lowest bidder. That has to stop. I understand that you want to demonstrate your expertise in the proposal. You can do this without giving your knowledge away.
In your next proposal include a sample of your expertise. That sample will be a previous solution to a different client. You can mark through the name and address so the customer remains anonymous. You now demonstrate your expertise, but can’t be shopped since the solution isn’t for that prospect.
If this problem happens once, shame on your customer for using you like that. If it happens more than once, shame on you for allowing it.